Filter presses as commonly utilized in the clay, chemical, or mining industries employ large areas of a textile material to form a filtering screen. The textile material is typically attached to a tubular hub that allows the material to be filtered to pass through successive filtering chambers in the press. The tubular hub must be securely attached to the filtering material to prevent leakage, and is usually constructed of a textile tube or a thermoplastic or rubber material.
Conventional methods for joining the filtering material to the thermoplastic tubular hub include sewing the parts together. This approach is labor intensive and thus expensive. Moreover, the sewing tends to create weaknesses in the join due to the stitch holes. Other methods include adhesive and bonding agents to secure the filtering cloth to the tubular hub, but these techniques are time consuming since the adhesive must set to form the bond, and is not consistently reliable.
U.S. Pat. No. 4,765,859 describes a method of joining filter cloth to a rubber tubular hub for use as a filter screen in a filtering press. The hub consists of a cylindrical tube that includes two annular flanges protruding radially outward from the circumference of the tube at each end. The hub is fabricated from a rubber material that is collapsible. A filtering screen is then constructed by joining two filter cloths to the hub at each end of the tube. One filter cloth is joined to the hub at the inside surface of one flange, while the other filter cloth is joined to the inside surface of the other flange. The filter cloth and hub are bonded together by induction heating to form a seal to prevent leakage of the filtered material.
The filter is manufactured by forming a circular hole in each cloth. Each circular hole has a diameter that is smaller than the diameter of the outer circle formed by each protruding flange so as to overlap the inside perimeter of each flange when positioned. To locate each filter cloth within the inside space between the flanges, at least one end of the tubular hub must be collapsed to allow each of the filter cloths to fit over the flange and be contained within. A pre-formed metal filled polypropylene ring or strand is placed between the filter cloth and the protruding flange. This assembly rests upon a split copper ring that is capable of generating a magnetic field at RF frequencies when electrically activated. Two split ring presser coils or half collars are placed between the flanges to pressure the inside surface of the flange, the polypropylene ring and the filter cloth together in forced contact. A flat presser plate is mounted on a vertical cylinder to clamp the flange, the polypropylene ring and the cloth together between the presser coils and the plate. When the bottom coil is electrically activated heat generated by RF induction heating of the metal particles within the polypropylene ring causes the polypropylene to soften. The clamping force presses the softened polypropylene into the filter cloth and the flange to form a lap joint. After the electrical activation cycle, the part is held clamped for a cooling cycle to allow the flange and filter cloth to bond together.
While U.S. Pat. No. 4,765,859 describes a clear advance over the prior art, there are certain disadvantages associated with the process. The process is relatively time consuming since the copper rings and collars must be manually fitted between the flanges of the hub. Furthermore, a flat weld requires sufficient cooling time before removal from the clamps to ensure good adhesion, and if removed prematurely the flat lap joint may be weakened. Consequently, the process significantly slows the manufacturing time, and is not compatible with an automated process.
A gap is necessarily created between the abutting collars, allowing the heated material to flow into the gap. The heated material that flows through the cloth into the gap weakens the cloth, and creates a point of likely failure during operation of the filter. As part of the process "weld" material is squeezed to the outside edge of the flange and overflows onto the circumference or lip of the flange. When cooled the "weld" material, mixed with the metal particles, is naturally abrasive, and any flexing of the filter cloth against this abrasive edge will eventually be a cause of failure.
The process is only useful in bonding like materials for the hub and filter cloth. Incompatible materials will not join with sufficient strength for filtering applications. Moreover, lighter materials are prone to a weakened join at the inside or outside edge of the join.
The process further requires that the hub be constructed of a flexible material so that it can be collapsed to allow the filter cloth to fit over it. The choice of materials that can be used for the hub is thus restricted, since a rigid material will likely crack or deform under the compression.
It is thus desirable to find a new method for securely joining a mesh material such as a cloth fabric or synthetic material, as commonly used in filtering applications, to a hub that eliminates the above stated disadvantages of the prior art. More specifically, it is desirable that the process eliminate or at least reduce the labor intensive aspects of known processes, in order to improve productivity and manufacturing efficiency. It is further desirable that the process create a reliable and secure bond between the filtering cloth and the hub so that the filtering material will not separate from the hub in a filtering operation.